What is the correct order of reactivity of alcohols toward reactions involving cleavage of C-O bond?

3°>2°>1°

The correct answer is option 2. 3°>2°>1°.

Let us delveinto why the reactivity of alcohols toward reactions involving the cleavage of the C-O bond follows the order: Tertiary (3°) > Secondary (2°) > Primary (1°)

Mechanism of Cleavage Reaction

The cleavage of the C-O bond in alcohols typically involves the following steps:

Formation of Carbocation:

The alcohol loses a water molecule (in dehydration or similar reactions), forming a carbocation. The stability of this carbocation is crucial as it determines the ease of the cleavage reaction.

Rearrangement and Reaction:  The carbocation may undergo further rearrangement or react with other species depending on the reaction conditions.

Carbocation Stability

The stability of the carbocation directly impacts the reactivity of the alcohol. Here’s why:

Tertiary Carbocations (3°):

Tertiary carbocations are stabilized by the inductive effects and hyperconjugation from three alkyl groups attached to the positively charged carbon. The electron-donating effects of these groups help stabilize the positive charge on the carbocation. Due to their high stability, tertiary alcohols readily form tertiary carbocations when the C-O bond is cleaved. This makes tertiary alcohols the most reactive in such reactions.

Secondary Carbocations (2°):

Secondary carbocations are stabilized by the inductive effects and hyperconjugation from two alkyl groups. They are more stable than primary carbocations but less stable than tertiary ones. Secondary alcohols form secondary carbocations upon cleavage, which is less favorable compared to tertiary alcohols but more favorable than primary alcohols. This moderate stability translates to moderate reactivity.

Primary Carbocations (1°):

Primary carbocations are the least stable due to having only one alkyl group to stabilize the positive charge. They are highly unstable and prone to rearrangement or other reactions before they can be formed. Primary alcohols are the least reactive toward cleavage reactions because forming a primary carbocation is less favorable. The C-O bond cleavage is thus less readily achieved in primary alcohols.

Summary of Reactivity Order

Tertiary Alcohols (3°): Highest reactivity because they form the most stable tertiary carbocations.

Secondary Alcohols (2°): Intermediate reactivity due to the formation of less stable secondary carbocations.

Primary Alcohols (1°): Lowest reactivity due to the formation of the least stable primary carbocations.

This understanding of carbocation stability explains why the correct order of reactivity for alcohols in reactions involving cleavage of the C-O bond is: Tertiary (3°) > Secondary (2°) > Primary (1°)